The present invention relates to an optical recording medium reproducing apparatus, and in particular, to an optical, recording medium reproducing apparatus adapted to reproduce optically recorded information recorded on an optical recording medium by irradiating the medium with a light spot of a laser beam.
Compact discs (CDs) have conventionally been used as optical recording medium reproducing apparatus of this kind, wherein the disc as an optical recording medium is irradiated via an optical system 0.45 in numerical aperture (NA) with a laser beam 780 [nm] in wave length which is generated by a laser diode.
In recent years, however, new laser beam sources with a wave length smaller than 780 [nm] (for example, red=680 [nm], and semiconductor lasers of green and blue) have been developed as light sources for optical recording medium reproducing apparatuses. These new laser beam sources serve to implement recording medium reproducing apparatuses that can reproduce recording media with a higher recording density than compact discs. These recording medium reproducing apparatuses that can reproduce recording media with a higher recording density than compact discs desirably have a compatible reproducing function that also enables conventional compact discs (CDs) to be reproduced.
The diameter of the light spot that can be formed by a laser beam of a large wave length is selected so as to be somewhat larger than the width W1 of pits P1 formed in the compact disc as recorded information as shown by reference L1 in FIG. 1(A). This allows the light spot L1 to constantly lie across the width W1 during movement when entering the pit P1 from land to scan it.
A sum signal (hereafter referred to as an xe2x80x9cRF signalxe2x80x9d) can thus be obtained from an optical pickup based on light reflected from the compact disc because of the light spot L1 in FIG. 1 (A). The RF signal falls from a first signal level LV11 to a second signal level LV12 when the light spot L1 passes through the end of the pit P1, and subsequently maintains the signal level LV12 until the light spot L1 has passed the pit P1 with itself lying across the pit P1, as shown in FIG. 1(B). This results in a sum signal in which the signal level changes in response to the lengths of the land and the pit P1 because the light spot L1 scans both the land and the pit P1.
Thus, the signal level of the RF signal RF1 decreases during scanning due to the interference between light reflected from the pit P1 and light reflected from a reflecting surface (the land) located around the pit P1. This also occurs when a light spot L2 of a relatively small wave length scans a pit P2 formed in an optical disc that has a higher recording density than the compact discs.
Each time the light spot L2 formed by a laser beam of a relatively small wave length enters the pit P2, an RF signal RF2, the signal level of which changes from LV21 to LV22 in response to recorded information can be obtained as shown in FIG. 2(B).
The light spot L2 formed by a laser beam of a relatively small wave length can be converged on a small diameter compared to the light spot L1 formed by a laser beam of a relatively large wave length, so the width W2 of the pit P2 may be smaller than the width W1 of the pit P1. As a result, optical recording medium reproducing apparatuses using the light spot L2 formed by a laser beam of a relatively small wave length deal with a high recording density, while optical recording medium reproducing apparatuses using the light spot L1 formed by a laser beam of a relatively large wave length cope with a low recording density.
If a high recording density optical recording medium reproducing apparatus is used to directly reproduce a compact disc (CD) designed to be reproduced by a low recording density optical recording medium reproducing apparatus, the diameter of the light spot L2 is smaller than or equal to the width W1 of the pit P1, as shown in FIG. 3(A). In this case, light reflected from the land and light reflected from the pit P1 interfere with each other when the light spot L2 enters and leaves the pit P1, whereas no interference occurs while the light spot L2 is scanning the pit P1 because it is totally included within the pit P1. The signal level of an RF signal RF3 changes from LV31 to LV32 only at both ends of the pit P1, as shown in FIG. 3(B).
Although the signals shown in FIGS. 1(B) and 2(B) can be detected by integral detection, the signal shown in FIG. 3(B) cannot be detected by such detection but requires differential detection. The differential detection, however, has a higher error rate than the integral detection.
This invention is proposed in view of the above points, and its objective is to provide an optical recording medium reproducing apparatus that performs reproducing operations using a light spot formed by a laser beam of a relatively small wave length and which can compatibly reproduce low recording density optical recording media.
This invention provides an optical recording medium reproducing apparatus that reproduces information recorded on an optical recording medium with a plurality of pits formed along tracks based on the recorded information, comprising: a laser beam emitting means for emitting a laser beam; a focus control means for controlling the focusing of the laser beam on the optical recording medium; and a control means for controlling the focus control means so as to increase the spot diameter of the laser beam emitted onto the optical recording medium when the medium has a low recording density with pits relatively sparsely arranged compared to the case in which the medium has a high recording density with pits relatively densely arranged.
Thus, when the optical recording medium mounted in the optical recording medium reproducing apparatus has a low recording density, this invention enables the information recorded in the tracks of the mounted low recording density recording medium to be reliably reproduced as in high recording density recording media, by using the control means to control the focus control means so as to increase the spot diameter of the laser beam.
According to this invention, to provide control in such a way that the spot diameter of the laser beam will be increased, the control means supplies different focus bias values depending upon whether the optical recording medium has a high or a low recording density.
In addition, this invention comprises a light receiving means for receiving a laser beam reflected from the optical recording medium; a servo error signal generating means for generating a servo error signal based on an output signal from the light receiving means; a polarity detecting means for detecting the polarity of the servo error signal; and a polarity selecting means for selecting the polarity of the servo error signal based on an output signal from the polarity detecting means, thereby reliably enabling the optical recording medium to enter a servo operation state even when the polarity of the servo error signal from the optical recording medium differs from that of standard optical recording media.
In addition, this invention comprises a light receiving means for receiving a laser beam reflected from the optical recording medium; a read (RF) signal generating means for generating a read (RF) signal for the recorded information based on an output signal from the light receiving means; a tangential push-pull signal generating means for generating a tangential push-pull signal based on the output signal from the light receiving means; an error detecting means for detecting the error conditions of the output signal from the light receiving means; and a selecting means for selectively outputting the read (RF) or tangential push-pull signal, wherein the control means controls the selecting means based on an output signal from the error detecting means. This constitution enables the selecting means to select the tangential push-pull signal (or a differential detection signal) in order to switch to a state in which the recorded information can be reliably reproduced, when no read (RF) signal (or integral detection signal) of a sufficient magnitude of signal level can be obtained to prevent the recorded information from being completely reproduced.
Furthermore, this invention comprises a light receiving means for receiving a laser beam reflected from the optical recording medium; an input read (RF) signal generating means for generating an input read (RF) signal for the recorded information based on an output signal from the light receiving means; a tracking error generating means for generating a tracking error signal based on the output signal from the light receiving means; an automatic level control means for providing control so as to maintain a constant signal level of the input read (RF) signal and outputting an output read (RF) signal; and a normalizing means for normalizing the signal level of the tracking error signal based on the signal level of the input read (RF) signal, thereby enabling the mounted optical recording medium to perform stable tracking operations even when the medium has an extremely high or low reflectance.
This invention thus employs an optical pickup that forms a light spot of a small wave length in order to reproduce an optical recording medium with a high recording density of recorded information, and can automatically set optimal reproducing conditions when a low recording density optical recording medium is mounted instead of the current high recording density optical recording medium, thereby enabling the implementation of an optical recording medium reproducing apparatus that can compatibly reproduce various optical recording media.